Volunteering to be part of a medical research study has risks and benefits. One risk most people don’t consider is the possibility that the trial will uncover some health problem you didn’t know you had.

That’s particularly relevant in research that involves advanced imaging technologies such as MRIs and CT scans, which provide detailed pictures of body organs and tissues. According to a new study, about 40% of volunteers participating in medical research at the Mayo Clinic learned about a previously unknown abnormality, or so-called incidental finding, that had nothing to do with the actual purpose of the study.

Among 1,376 volunteers who underwent 1,426 imaging exams in 91 different studies, CT scans of the abdomen and pelvis were the most likely to generate unexpected findings, followed by CT scans of the chest and then MRIs of the head. Ultrasound and X-rays produced fewer of these incidental findings, says study co-author Dr. Nicholas Orme, because they are not as detailed as the more modern scans. (More on Time.com:Clinical Trial Dilemma: Save Lives Now — or Later?)

“I was personally a little surprised that 40% of our imaging exams had at least one incidental finding,” he says. “It goes to show that when we do an image for a patient or a research subject, we will get other information that were weren’t necessarily looking for, and we need to know how to address that.”

Orme notes that in the majority of cases, the finding did not require any further follow up. Only 6% involved additional testing, of which 1% ended up providing some benefit to the patient, such as catching a cancer or signs of infection early. Most of the incidental findings involved buildup of calcium in large blood vessels, which could be a sign of atherosclerosis, a condition that nearly every middle-aged American adult has.

But the study raises interesting questions about how researchers should handle such unexpected results, which may occur more frequently as our imaging technologies get better and better at picking up the smallest abnormalities in the body. In an accompanying editorial, Dr. Bernard Lo from the program in medical ethics at University of California, San Francisco, proposes more explicit discussion between the researcher and research volunteers about the possibility of exposing incidental findings.

“My sense is this isn’t uniform practice. I don’t think everybody is always [fully informed about] incidental findings when they undergo a research study,” says Orme. “Overall we could probably do better at that in the research community.”

The current study, which identifies the types of scans and regions of the body that tend to lead to the most unexpected findings, might help guide review boards about which type of medical research deserves special attention and when to raise the issue with volunteers who are seeking to join a study.